Piston

ABSTRACT

A crosshead piston 10 comprises a head member 18 and a skirt member 20, each of which can be secured independently to a piston pin 14. The skirt 20 has a reduced diameter upper end portion 66 which fits within lower end 24 of the head member ring belt portion 23, thereby exposing maximum head member surface area to a cylinder or cylinder sleeve wall for cooling. Preferably, the skirt end portion 66 defines a seal groove 32 which contains a metal ring 67 for sealing the gap 62 between the head member and skirt member.

This application is a continuation of Ser. No. 334,959, filed 12/28/81now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates generally to pistons for internalcombustion engines and more particularly to two-piece or crosshead-typepistons.

Crosshead type pistons in which the head and skirt members are separateand independently connected to the piston pin are well known and arepreferred in many applications, such as in heavy duty two stroke dieselengines. One advantage of this type of piston is that the skirt membergenerally bears all of the side thrust applied to the piston through theconnecting rod, leaving the head member to transmit only axiallydirected forces from the combustion chamber. Another advantage is thatthe skirt member may be made of less expensive materials because it isnot exposed to the forces of the combustion chamber.

One problem associated with crosshead type pistons is excessive heatbuild-up, particularly in the head member which is exposed to thecombustion chamber. In one-piece pistons, heat from the piston crown canbe transferred directly to the piston skirt for dissemination. However,in crosshead pistons with the head member separate from the skirt, heatwhich otherwise could be directly transferred to the skirt can bedispersed no further than the ring belt section--the cylindrical portionof the head just below the crown which usually contains piston rings.The exterior surface of the ring belt section is exposed to the cylinderwall (typically a cylinder liner in heavy duty engines) which draws heatfrom the head member.

The head member heat build-up problem becomes more pronounced as thesize of the head is reduced. There is a tendency to make the head memberas small as possible because more expensive materials and/or processingis used in the head member.

The problem is further complicated because the head member, particularlythe ring belt section, usually has a reduced diameter section which fitsinto the skirt member, thereby reducing the area of the ring belt whichis exposed to the cylinder wall.

SUMMARY OF THE INVENTION

The present invention helps to alleviate the heat build-up problem incrosshead pistons by exposing more of the head member surface area tothe cylinder wall without increasing the size or amount of material usedin the head member. A unique sealing arrangement is also provided at thejunction of the head and skirt members.

More specifically, the present invention is an improved crosshead pistonwherein the skirt member has a reduced diameter upper end portion whichis positioned within the lower end portion of the head member.Preferably, the reduced diameter skirt portion has an annular groovewhich contains a seal ring in engagement with the lower end portion ofthe head member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially exploded cross-sectional view of the pistonassembly of the present invention; and

FIG. 2 is an enlarged cross-sectional view of the seal between the headand skirt members of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 illustrates a crosshead piston 10 along with a connecting rod 12and a piston pin 14. The piston includes a head member 18 and a skirtmember, or crosshead, 20, which are separately formed, preferably ofcast iron. The skirt 20 is formed of ductile cast iron while the head 18is formed of malleable cast iron, which is more expensive to process,but which is more suitable for exposure to a combustion chamber.

The upper portion of the piston head 18 comprises a crown section 22which forms a combustion chamber defining wall. Depending from the outeredge of the crown section is an outer cylindrical ring belt section 23in which are formed three compression ring grooves.

The piston head 18 further includes a piston pin connecting section 33comprising a pair of spaced downwardly (as viewed in FIG. 1) extendingpin encircling arms 34 having piston pin openings 42. The arms areinterconnected at their upper ends by a saddle 36 which is in turnconnected with the crown section by a plurality of axially and radiallyextending struts or columns 37. Columns 37 include radially outwardlyextending rib portions 38 integrally connecting the columns with thering belt section 23 and with the outer portions of the crown section22.

Saddle 36 defines a downwardly opening semicylindrical recess 40. Recess40 terminates circumferentially in a pair of radial shoulders 43 whichretain a semicylindrical bearing insert 44 having an inwardly facingbearing surface preferably containing a plurality of grooves (not shown)for the distribution of lubricant thereover. Oil supply openings 54 and55 are provided in the centers of the saddle portion and bearing insertrespectively.

The crosshead or skirt member 20 comprises generally a cylindrical wall57 having a pair of centrally located oppositely disposed, and inwardlyextending bosses 58. Each boss 58 defines a cylindrical pin receivingopening 60 which, when the head member and skirt member are assembled,are axially aligned with the pin encircling openings 42 of the headmember pin connecting section. At this lower end, skirt 20 has two oilring grooves 72 for suitable oil scraper rings. Below each groove 72 area plurality of oil drain holes 73 to carry away oil from the oil ringsin a conventional manner.

The head and skirt members of the piston assembly are retained togetherby the cylindrical piston pin 14 which is received in openings 60 and 42of the skirt and head members, respectively, and engages the bearinginsert 44 across a substantial portion of its upper surface. Theconnecting rod 12 is secured to the piston pin 14 by bolts (not shown)which extend through openings 84 in the connecting rod and 78 in the pinin a manner well known in the art.

Piston pin 14 is hollow, having a slightly thickened wall at its centerwhere oppositely disposed upper and lower openings 76 and 77,respectively, are provided. A tube 88 is aligned with openings 76 and 77to provide an oil passage through the pin. Tube 88 is aligned with adrilled oil passage 90 extending lengthwise through the connecting rod12. Metal closure plates 59 seal each end of the hollow pin 14 as iswell known in the art.

As more clearly shown in FIG. 2, the upper end of the skirt 20 has anenlarged wall 66 which is cast approximately three times as thick as theother areas of the skirt wall. The radially outer portion of wall 66 ismachined away to provide a reduced diameter end portion having acylindrical surface 64 defining a cylinder. The ring belt lower wall 24which receives the upper end skirt wall 66 is also machined as its lowerend portion to create an enlarged diameter interior cylindrical surface63. It should be noted that the head member 18 and the skirt member 20are designed to operate without contacting each other near the interfacearea shown in FIG. 2. Each member 18 and 20 is secured to the piston pinindependently, as previously described.

A narrow annular gap 62 exists between the inner cylindrical surface 63of the head member and the outer reduced diameter cylindrical surface 64of the skirt 20. This gap permits a slight rocking of the skirt 20 underthe angular piston rod forces but prevents these angular forces frombeing transferred thru the skirt to the head member 18. Beveled annularsurfaces 71 and 72 on the skirt and head members, respectively, aid inassembly and in the limited movement of the skirt.

The gap 62 is sealed by a cast iron sealing ring 67 which allowsrelative sliding or rocking movement between the head and skirt memberswithout deterioration of the ring 67 and without breaking the sealbetween the members. The ring 67 has a tight end gap in the range of0.003 inches, which is possible because the ring experiences little heatexpansion due to the piston design. The seal ring is received in theseal groove 32 which is cut into circumferential surface 64 of theskirt. It is widely preferred that the seal groove be cut into acircumferential surface such as 64 to allow ring 67 to engage acylindrical surface such as 63. This sealing arrangement is believed tobe more effective during any sliding or rocking between the head andskirt members than previously known designs which used elastomericsealing rings, or which relied upon seals between radial or conicalsurfaces.

It can be seen that the ring belt section 23, by virtue of the presentinvention in which the head member extends over the skirt member, islengthened and has an overlap or receiving portion 24, the externalsurface of which is exposed to a cylinder or cylinder sleeve wall 99,thereby allowing an associated cylinders or cylinder sleeve wall to drawmore heat from the head member. The additional exposed area is createdwithout increasing the size or the material used in the head, ascompared to similar designs in which the head member is received withinthe skirt.

Although the previously described embodiment of the invention ispreferred presently, numerous modifications or variations are within thescope of this invention as defined in the following claims.

What is claimed is:
 1. A crosshead piston assembly comprising:a skirtmember having means for drivingly engaging a piston pin and a radiallyouter cylindrical surface for close sliding association with a cylinderwall, a head member formed separately from said skirt member, said headmember having a crown for exposure to a combustion chamber, a radiallyouter cylindrical surface for close sliding association with saidcylinder wall, said skirt cylindrical surface being coaxial with saidhead cylindrical surface, and means for drivingly engaging a piston pinindependently of said skirt member; said skirt member having a reduceddiameter end portion defining a skirt cylinder, said skirt cylinderreceived within said head member; said head member having a radiallyinner cylindrical surface adjacent an end portion thereof defining ahead cylinder, said skirt cylinder received within said head cylinder,wherein said skirt cylinder and said head cylinder are substantiallycoaxial and wherein said skirt cylinder diameter is less than said headcylinder diameter, whereby a cylindrical gap exists between said skirtcylinder and said head cylinder; said skirt member having a generallyradially extending annular surface between said skirt cylinder and saidskirt member radially outer cylindrical surface, wherein said headmember end portion comprises a generally radially extending annularsurface positioned coaxially and adjacent said radially extendingannular skirt surface, wherein a generally radially extending annulargap exists between said generally radially extending head and skirtsurfaces and adjacent said cylindrical gap; and an annular seal betweensaid skirt cylinder and said head cylinder.
 2. The assembly as definedin claim 1 further comprising an annular seal groove in one of saidskirt cylinder and said head cylinder, said seal positioned in saidgroove and in engagement with the other of said skirt cylinder and saidhead cylinder to form a seal between said skirt cylinder and said headmember.
 3. The assembly as defined in claim 2 wherein said seal grooveis in said skirt cylinder.
 4. The assembly as defined in claim 3 whereinsaid head cylinder has an enlarged diameter for receiving said skirtcylinder, wherein said seal is engaged with said enlarged diameter. 5.The assembly as defined in claim 4 wherein said seal is an annular metalring generally rectangular in cross-section, wherein the outercylindrical surface of said ring engages said head cylinder.
 6. Theassembly as defined in claim 5 wherein said head cylinder comprises afrusto-conical radially inner surface for engaging said rectangularcross-section metal ring during assembly, wherey said metal ring isbiased radially inwardly by said frusto-conical surface during assembly.